Opto-couplers are used in switched mode power supplies (SMPS) and other systems that require mains isolated communications. Systems may be isolated from a mains power supply in order to protect the system from any surge events that may affect the mains supply. Mains isolation is typically used to protect people, and every electronic device that is powered from the mains supply needs isolation. If there are no metal parts that can be touched, the electronics does not need mains isolation as the isolation is taken care of by the (plastic) casing. If there are metal parts like external connections on notebook charger output plugs, television scart cables, or computer usb connections for example, then the electronics needs isolation from the mains. If electronic communication between the mains isolated side and the non-isolated side is required, then in many cases opto-couplers are used.
An opto-coupler typically contains an infra-red light emitting diode (LED) and a photo-detector, as is known in the art. Typically the diode voltage of the opto-coupler is about 1 volt. In some systems, a much higher voltage is used for controlling the opto-coupler, and the excess voltage can be dropped across a linear regulator and/or a series resistor.
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